Thread: PostgreSQL 18 Beta 1 io_max_concurrency

Hello,

The default setting of parameter io_max_concurrency shows 64 whereas the documentation says -1, checking if this is the expected behavior.

PG 18 documentation says:

The default setting of -1 selects a number based on shared_buffers and the maximum number of processes (max_connections, autovacuum_worker_slots, max_worker_processes and max_wal_senders), but not more than 64.

https://www.postgresql.org/docs/18/runtime-config-resource.html#GUC-IO-MAX-CONCURRENCY

postgresql.conf (also says -1)

#io_max_concurrency = -1 # Max number of IOs that one process

                         # can execute simultaneously

     # -1 sets based on shared_buffers # (change requires restart)

But the default setting in the database instance is  64

postgres=# show io_max_concurrency;

io_max_concurrency

--------------------

64

(1 row)

Even after explicitly changing to -1 and restart, the setting is not getting reflected, is there any other dependency we could be missing?

postgres=# alter system set io_max_concurrency=-1;

ALTER SYSTEM

waiting for server to shut down.... done

server stopped

waiting for server to start.... done

server started

[postgres]$ psql

psql (18beta1)

Type "help" for help.

postgres=# show io_max_concurrency;

io_max_concurrency

--------------------

64

(1 row)

Interestingly, it accepts values besides -1, including unexpected ones like 100/200 (contrary to the documentation max 64).

postgres=# show io_max_concurrency;

io_max_concurrency

--------------------

100

(1 row)

postgres=# alter system set io_max_concurrency=200;

ALTER SYSTEM

postgres=#

waiting for server to shut down.... done

server stopped

waiting for server to start.... done

server started

[postgres]$ psql

psql (18beta1)

Type "help" for help.

postgres=# show io_max_concurrency;

io_max_concurrency

--------------------

200

(1 row)

Thank you,

Nazneen